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<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width,initial-scale=1,maximum-scale=1,minimum-scale=1,user-scalable=no">
    <title>Mutation Observer</title>
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<body>
    <h1>Mutation Observer</h1>
    <button id="btn">click</button>

    <script src="main.js"></script>
</body>
</html>
